1. Field of the Invention
The present invention relates to parachute designs, particularly to the cruciform-type parachute design.
2. Description of the Prior Art
Parachutes are an integral component of systems used to deliver cargo or loads aerially to remote or inaccessible locations. To deliver a load aerially, the load, furnished with a parachute delivery system, is transported to the delivery site by aircraft. Upon reaching the delivery site, the load is released, ejected or dropped from the aircraft. Shortly after release, a parachute is deployed which is attached to the load by suspension lines and other rigging. The deployed parachute decelerates the descending load to a velocity at which the load may land on the ground or water without damage.
Desirable features for an aerial-delivery parachute include low manufacturing costs. Another desirable feature is a high drag coefficient, which is related to manufacturing cost, as a parachute design with a higher drag coefficient can be smaller or have less material in its construction to deliver the same load. Another desirable feature is strong ballistic characteristics, without a tendency to glide randomly, so that the load does not collide with other aerially-delivered cargoes. A further desirable feature is high stability, since a ballistic load in aerial delivery has no means of controlling its descent once the parachute has deployed.
Several parachute designs have traditionally been used for aerial delivery. One is referred to as the radial-gore circular parachute. This parachute design is the one commonly associated with paratroopers. It is constructed of panels, or gores, each cut in a generally slender, tapering, trapezoidal shape and sewn together along their elongated sides to form a circular pattern, with the narrow base of each gore at the center of the circular pattern and the longer base at the perimeter, forming the skirt of the parachute.
A radial-gore circular parachute has a high drag coefficient and is generally stable. However, it has high manufacturing costs, due to the material wasted from cutting the gores and from the large number of seams to be sewn.
Examples of variations of this radial-gore circular parachute design include Thomblad, U.S. Pat. No. 1,685,688 (1928); Henvis, U.S. Pat. No. 1,733,732 (1929); Malmer, U.S. Pat. No. 1,777,441 (1930); Sedmayr, U.S. Pat. No. 2,119,183 (1938) and Hart, U.S. Pat. No. 2,458,264 (1949).
Another parachute design used in aerial cargo delivery is the cruciform-type parachute. The cruciform-type parachute, when viewed from above or stretched out deflated, resembles a cruciform, or cross shape. In its simplest embodiment, it is formed by orthogonally overlapping two equal rectangles of material, with each rectangle having an aspect ratio of approximately 1:3, and sewing around the perimeter of the square overlapping area. This creates a design with a square center panel, which in this fabrication method would be two plies thick, with four square or rectilinear single-ply side panels conjoined on each edge of the square center panel. Suspension lines are attached to the edges of the side panels opposite from the square center panel, which conjoin at their opposite ends at the load. Using two rectangular panels with aspect ratios less than 1:3 will produce a design with a square center panel and rectangular side panels.
The cruciform-type parachute is simple and inexpensive to fabricate, involving little material waste in fabricating the two rectangular panels, and low labor costs due to the few number of seams in the design. However, the typical cruciform-type parachute design has low drag efficiency and low stability.
Typical examples of variations on the cruciform-type parachutes can be found in Mitchell, U.S. Pat. No. 3,531,067; Slater et al., U.S. Pat. No. 3,602,462; and Engel, U.S. Pat. No. 3,741,505.
As can be seen in these examples, the side panels are pulled towards a more spherical shape by the suspension lines connecting the lower edges of the side panels, often referred as the skirt, to the suspended cargo, load or parachutist. This causes the skirt to cup or curl as it is drawn inward by the suspension lines. The side edges of the side panels are unconstrained, allowing significant undesirable air leakage, as well as causing turbulence which can destabilize the descending parachute.
Several attempts have been made in the art to control the unconstrained lateral edges of the side panels of these cruciform-type parachutes. For example, in Berland, U.S. Pat. No. 6,443,396 (2002), the lateral edges of the side panels are stitched together. However, this creates a parachute with a more polyhedral rather than a spherical shape, which in turn leads to uneven levels of stress and strain throughout the parachute.